1. Field of the Invention
The present invention relates to a wireless communication system and more particularly to a method of handover in a wireless communication system.
2. Discussion of the Related Art
There are hard handover (HHO) and soft handover in methods of handover according to related art. And there are Macro Diversity Handover (MDHO) and Fast Base Station Switching (FBSS) in soft handover. However, soft handover has large overhead. Thus, technology about hard handover is studied.
FIG. 1 illustrates procedure of hard handover according to related art. In FIG. 1, a serving base station (SBS) is what a mobile station is served by and a target base station (TBS) is what a mobile station is going to perform a handover to.
In hard handover according to related art, a mobile station previously synchronizes with a target base station to reduce communication interruption time during hard handover.
As illustrated in FIG. 1, a mobile station transmits a mobile station handover request (MOB_MSHO-REQ) message to a serving base station S110. Then, a serving base station transmits a handover request (HO-REQ) message to a target base station S120, and receives a handover response (HO_RSP) message S130. And, a serving base station transmits a handover response (MOB_BSHO_RSP) message to a mobile station as a response of the MOB_MSHO-REQ message of a mobile station S140. A mobile station acquires information of a target base station through the MOB_BSHO_RSP message, and prepares a previous procedure for handover, and transmits a handover indicate (MOB_HO_IND) message to a serving base station.
A base station changes communication setting according to a target base station, and waits for a Fast_Ranging_IE (information element) until action time. The Fast_Ranging_IE includes information for ranging process between a mobile station and a target base station. Action time is a time at which a target base station transmits the Fast_Ranging_IE to a mobile station.
After receiving the Fast_Ranging_IE, a mobile station performs an authentication and reopens communication.
A mobile station, a serving base station and a target base station buffer data to prevent loss of data during communication interruption, transmits and receiver the buffed data and continue communication. A mobile station starts buffering from transmitting the MOB_MSHO-REQ message, and a serving base station starts buffering from receiving the MOB_MSHO-REQ message.
Recently, technologies about femto cell for supporting indoor or shadow area which macro cell cannot support are being defined. A femto cell base station is a micromini and low power indoor base station. Femto cell is a coverage of a femto cell base station and similar concept with pico cell. Femto cell is evolved more than pico cell. A femto cell base station is a mini cellular base station connected to a broadband router and connects data to backbone network of a mobile communication company trough Digital Subscriber Line (DSL).
So, appearance of a femto cell base station makes method of handover to a femto cell base station necessary.